A composite-type mixed oxygen ion and electronic conductor having mixed electrical conductivity arising from oxygen ions and electrons has simultaneously high oxygen ion conductivity and electronic conductivity. In order to apply this substance to the above-mentioned fields, such properties are required as that high mixed electrical conductivity must be maintained in the range from high partial oxygen pressure such as air to extremely low partial oxygen pressure such as hydrogen gas or methane gas, and also chemically and mechanically stable in temperature range of 500-1000° C. where it is generally used.
The mixed conductor having mixed electrical conductivity arising from oxygen ions and electrons (the mixed conductor) has two types as shown below in wide classification.
(1) Single-Phase Mixed Conductor
This is the type that the substance itself has mixed conductivity, and as examples, Perovskite-type oxides in La—Sr—Co—Fe, and La—Sr—Ga—Fe systems are well known (See Ref.: Chem. Lett., 1985, 1743, Am. Ceram. Soc. Bull., 74, 1995, 71, Catal. Today, 36, 1997, 265, Solid State Ionics, 129, 2000, 285, Solid State Ionics, 135, 2000, 631.).
(2) Composite-Type Mixed Conductor
This is the type in which mixed conductivity is realized by making a composite from two substances one of which shows, as a single component, oxygen ion conductivity, and the other of which shows, as a single component, electronic conductivity, and for examples, the composite of an ionic conductor such as yttria-stabilized zirconia (YSZ) and an electronic conductor such as metal Pd, or the composite of gadolinium-doped cerium oxide and Perovskite-type oxides in La—Sr—Mn system are known.
In case of a single-phase mixed conductor, the substances are extremely limited to obtain high mixed conductivity, while for a composite-type mixed conductor wide varieties of substances are applicable. Also in case of composite-type mixed conductors, since several other species of substances are able to be included, it is possible to impart secondary functions, such as catalytic function.
However, composite-type mixed conductors have their own problems. For example, in YSZ-Pd composite, since one component is a ceramics while the other is a metal, it is difficult to obtain densified sintered product, and even if it is obtained, in case of the thermal expansion coefficients widely different in the two phases, cracking may be caused by thermal cycles or other like phenomena.
Further, it is necessary to form conductive networks of oxygen ion conductive phase and electronic conductive phase, respectively, in order to realize mixed conductivity, so that it is required to take into consideration the problem of percolation based on the volume ratio of the phases. Here, percolation is such a phenomenon that, for example, when metal powders are mixed into insulation matrix to obtain electrical conductivity, if the volume ratio of metal powders to insulation matrix exceeds a certain value (critical volume ratio), the network of metal powder is formed in the insulator, and the conductivity increases suddenly.
It is also pointed out for the composite of gadolinium-doped cerium oxide and Perovskite-type oxides in La—Sr—Mn system that, even when high mixed conductivity is attained, the two phases cause chemical reaction upon using at high temperature, and a different phase is formed to disturb mixed conductivity at interface (See Ref.: Solid State Ionics 146, 2002, 163, and J. Eur. Ceram. Soc. 21, 2001, 1763.).
In view of the above-mentioned problems, it is the first object of the present invention to provide a composite-type mixed oxygen ion and electronic conductor, in which oxygen ion conductive phase and electronic conductive phase are uniformly mixed, thereby cracking due to the difference of thermal expansion coefficient does not occur, oxygen ion conductive phase and electronic conductive phase respectively form conductive networks, and a different phase to disturb mixed conductivity is not formed by the reaction of both phases, consequently oxygen permeability is high, and aged deterioration does not easily occur. In addition, it is the second object to provide the method to manufacture said composite-type mixed oxygen ion and electronic conductor.